Recently, energy saving in automobiles and construction or agricultural machinery, i.e., fuel saving has become an urgent need in order to deal with environmental issues such as reduction in carbon dioxide emissions, and units such as engines, transmissions, final reduction gears, compressors, or hydraulic power units have been strongly demanded to contribute to the energy saving. Consequently, the lubricating oils used in these systems are required to reduce stir resistance and frictional resistance more than before.
Reduction of the viscosity of a lubricating oil used in a transmission and a final reduction gear unit can be exemplified as an effective energy saving means. For example, a manual transmission or a final reduction gear unit has a gear bearing mechanism. Reduction of the viscosity of a lubricating oil to be used therein can reduce the stir and frictional resistances and thus enhance the power transmission efficiency, resulting in an improvement in the fuel efficiency of an automobile.
However, reduction of the viscosity of the lubricating oil used in these transmissions and units may cause the above-described units and mechanisms thereof to be significantly shortened in fatigue life or reduced in extreme pressure properties and may generate seizure, possibly resulting in some malfunctions in the transmissions. In particular when a low viscosity lubricating oil is blended with a phosphorus-based extreme pressure additive to enhance the extreme pressure properties of gears, the fatigue life thereof will be extremely shortened. In general it is thus difficult to reduce the viscosity of the lubricating oil. Alternatively, a viscosity index improver can improve the viscosity characteristics of a lubricating oil at low temperatures or practical temperatures but is not generally expected to improve the fatigue life or extreme pressure properties but also known to cause the viscosity to be reduced due to shear occurring during the long time use when the viscosity index improver is used in a lubrication oil for transmissions.
Examples of conventional automobile transmission oils which enable a transmission to maintain various properties such as shifting properties for a long time include those produced by optimizing and blending synthetic and/or mineral base oils, antiwear agents, extreme pressure additives, metallic detergents, ashless dispersants, friction modifiers and viscosity index improvers (for example, see Patent Literature Nos. 1 to 3 below).
However, these compositions do not aim at improving the fuel efficiency of an automobile and thus are high in kinematic viscosity. Any of the documents does not at all refers to the influences on fatigue life or extreme pressure properties in an early stage and after a long time use when the lubricating composition is lowered in viscosity. Therefore, a composition which can solve the foregoing problems has not been sufficiently studied yet.